Hydroxyl group-containing diene rubber

ABSTRACT

The present invention relates to rubber mixtures containing diene rubber with a concentration of primary hydroxyl groups of 0.1 to 2 wt. % and a glass transition temperature of −120° to −50° C. and their mixtures with fillers, optionally further rubbers and rubber auxiliary substances and vulcanisates prepared therefrom. Rubber mixtures according to the invention are suitable for producing highly reinforced, abrasion-resistant moulded items, in particular for producing tires treads which have a particularly high resistance to wet skidding.

FIELD OF THE INVENTION

The present invention provides rubber mixtures which contain a dienerubber with a concentration of primary hydroxyl groups of 0.1 to 2 wt. %and a glass transition temperature of −120 to −50° C. and their mixtureswith fillers, optionally other rubbers and rubber auxiliary substancesand vulcanisates prepared therefrom. Rubber mixtures according to theinvention are suitable for producing highly reinforced,abrasion-resistant moulded items, in particular for producing tireswhich have particularly high wet skidding resistance.

BACKGROUND OF THE INVENTION

Double bond-containing solution rubbers, such as solution polybutadieneand solution styrene/butadiene rubbers, have advantages over thecorresponding emulsion rubbers when producing low rolling resistancetire treads. The advantages are based, inter alia, on the ability tocontrol the vinyl content, cis content and glass transition temperatureand molecular branching associated therewith. Particular advantages inrelation to abrasion, wet skidding resistance and rolling resistance ofthe tires result therefrom in a practical application. Thus, U.S. Pat.No. 5,227,425 describes the production of tire treads from a solutionSBR rubber and silica. To further improve the properties, numerousmethods for modifying the end groups have been developed, as isdescribed e.g. in EP-A 334 042, with dimethylaminopropyl-acrylamide or,as described in EP-A 447,066, with silyl ethers. However, due to thehigh molecular weight of the rubber, the proportion by weight of endgroups is small and can therefore have only a small effect on theinteraction between filler and rubber molecule. The present invention isintended to provide solution diene rubbers such as solutionpolybutadiene and polyisoprene with a much higher concentration ofeffective groups for interacting with fillers.

A process for preparing hydroxyl group-containing solution polybutadienerubbers is also described in DE-OS 2,653,144. These rubbers, however,have a much higher concentration of hydroxyl groups, associated withmuch higher glass transition temperatures.

EP-A 464,478 describes a process for hydroxylating rubber, wherein,however, secondary hydroxyl groups are introduced which are far lesseffective than the primary hydroxyl groups in the present invention.

Hydroxyl group-containing emulsion and solution rubbers are alsodescribed in EP-805,452 A1, wherein the hydroxyl concentrationsdescribed here for solution rubbers lie within a much lower range (0.009to 0.061%), depending on the process used, and the glass transitiontemperatures are substantially higher (>−40° C.), depending on thestyrene content, for the emulsion rubbers described.

SUMMARY OF THE INVENTION

It has now been found that rubber mixtures and rubber vulcanisates withsurprisingly improved dynamic damping characteristics in the temperaturerange relevant to wet skidding resistance and the temperature rangerelevant to rolling resistance and also improved abrasion behaviour canbe prepared from hydroxyl group-containing solution diene rubbers with aconcentration of 0.1 to 2 wt. % of bonded primary hydroxyl groups and aglass transition temperature of −120 to −50° C. Further surprisingadvantages were obtained when the rubber mixture was prepared not inconventional compounders but by mixing a solution of hydroxylgroup-containing rubber and oxidic or siliceous fillers in organicsolvents and the solvent was then removed using steam, since then thefiller is fully precipitated with the rubber and does not remain in thewaste water, as when using an unmodified rubber.

DETAILED DESCRIPTION OF THE INVENTION

The present invention therefore provides rubber mixtures containing oneor more hydroxyl group-containing rubbers built up from diolefins,characterised in that the hydroxyl group-containing rubber(s) contain inthe range from 0.1 to 2 wt. % of bonded primary hydroxyl groups and havea glass transition temperature between −120 and −50° C. and fillers. Theinvention also provides use of said rubber mixtures for preparing rubbervulcanisates, in particular silica-filled tire treads with especiallyhigh abrasion resistance, especially high wet skidding resistance and alow rolling resistance.

Suitable diolefins are in particular 1,3-butadiene, isoprene,1,3-pentadiene, 2,3-dimethylbutadiene, 1-phenyl-1,3-butadiene and/or1,3-hexadiene. 1,3-butadiene and isoprene are particularly preferablyused.

Rubbers according to the invention for use in rubber mixtures accordingto the invention can be prepared preferably by polymerisation usingcoordination catalysts in the presence of a solvent or by anionicsolution polymerisation. Coordination catalysts in this connection areunderstood to be Ziegler-Natta catalysts, coordination catalysts andmonometallic catalyst systems. Coordination catalysts are preferablythose based on Ni, Co, Ti or Nd. Catalysts for anionic solutionpolymerisation are based on alkali or alkaline earth metals such as e.g.n-butyllithium. In addition, known randomised control agents for themicrostructure of the polymer may be used. These types of solutionpolymerisations are known and are described e.g. in I. Franta Elastomersand Rubber Compounding Materials; Elsevier 1989, pages 113-131 and inHouben-Weyl, Methoden der Organischen Chemie, Thieme Verlag, Stuttgart,1961, vol. XIV/1 pages 645 to 673 or in vol. E20 (1987), pages 114 to134 and pages 134 to 153. The primary hydroxyl groups are introduced ina subsequent reaction of the final polymer. Methods for introducingprimary hydroxyl groups are e.g. the addition of primary hydroxylgroup-containing mercaptans, an addition reaction with formaldehyde,reaction with carbon monoxide followed by hydrogenation, hydroborationfollowed by oxidative hydrolysis of the borane compound.

The hydroxyl groups are preferably introduced by the addition ofhydroxylmercaptans of the general formula (1) and/or hydroxylgroup-containing mercaptocarboxylic esters of the general formula (2).

HS—R¹—OH  (1)

HS—(CHR²)_(n)—(CO₂—R³—OH)_(m)  (2)

in which

R¹ represents a linear, branched or cyclic C₁-C₃₆ alkylene group, whichmay optionally be substituted with up to 6 further hydroxyl groups, orwhich may be interrupted by nitrogen, oxygen or sulfur atoms,

R² represents hydrogen or a C₁-C₆ alkyl group and R³ represents alinear, branched or cyclic C₂-C₃₆ alkylene group, which may optionallybe substituted with up to 6 further hydroxyl groups or may beinterrupted by nitrogen, oxygen or sulfur atoms,

OH represents a primary hydroxyl group,

n is an integer from 1 to 5

m is an integer from 1 to 2.

C₁-C₃₆ alkylene groups are understood to be any linear, cyclic orbranched alkylene groups with 1 to 36 carbon atoms which are known to aperson skilled in the art, such as methylene, ethylene, n-propylene,i-propylene, n-butylene, i-butylene, t-butylene, n-pentylene,i-pentylene, neo-pentylene, n-hexylene, cyclohexylene, i-hexylene,heptylene, octylene, nonylene decylene, undecylene and dodecylene.

Preferred hydroxylmercaptans are mercaptoethanol, 1-mercapto-3-propanol,1-mercapto-butanol, 3-mercapto-1,2-propanediol (thioglycerol),α-mercapto-ω-hydroxy-oligoethylene oxides such as e.g.α-mercapto-ω-hydroxyoctaethylene glycol or the corresponding ethyleneoxide/propylene oxide mixed polyether. Mercaptoethanol, thioglycerol andα-mercapto-ω-hydroxy-oligoethylene oxides are particularly preferred.

Preferred hydroxyl group-containing mercaptocarboxylic esters are estersof mercaptoacetic acid, mercaptopropionic acid and mercaptobutyric acidwith ethylene glycol, propylene glycol, butylene glycol, diethyleneglycol, triethylene glycol, tetraethylene glycol, octaethylene glycol,dipropylene glycol, tripropylene glycol, tetrapropylene glycol,glycerol, N-methyl-diethanolamine. The corresponding esters ofmercaptoaectic acid and 3-mercaptopropionic acid are particularlypreferred.

Suitable radical starters for the addition of hydroxylmercaptans tohydroxyl group-containing rubbers arc e.g. azo-initiators such asazobisisobutyronitrile, azobiscyclohexanonitrile and peroxides such asdilauroyl peroxide, benzopinacolsilyl ether or photoinitiators in thepresence of UV or visible light. Preferred radical starters are di-acylperoxides, in particular dilauroyl peroxide, didocecanoyl peroxide,di-(3,3,5-trimethylhexanoyl) peroxide, disuccinoyl peroxide, dibenzoylperoxide and perketals such as1,1-di-(tert.-butylperoxy)-3,3,5-trimethylcyclohexane,1,1-di-(tert.-butylperoxy)-cyclohexane and1,1-di-(tert.-butylperoxy)-butane.

Preferred amounts of radical starters are 0.5 to 20 wt. %, with respectto the hydroxyl-mercaptan.

The average molecular weight of the hydroxyl group-containing rubbers isbetween 50,000 and 2,000,000, preferably between 100,000 and 1,000,000.

The Mooney viscosity ML 1+4 of the copolymers is between 10 and 200,preferably 30 and 150, measured at 100° C.

The concentration of copolymerised 1,2-butadiene units (“vinyl content”)is between 0 and 60 wt. %, preferably 1 to 50 wt. %.

The glass transition temperature is between −120 and −50° C., preferably−120 and −70° C. The glass transition temperature can be determinedusing known methods, e.g. by means of DSC (differential scanningcalorimetry, rate of heating 20 K/min).

The cis-1,4 content of copolymerised dienes is between 10 and 100%,preferably between 30 and 99%, in particular 90-99%.

The concentration of primary hydroxyl groups is between 0.1 and 2 wt. %,preferably in the range 0.1 to 1 wt. %, in particular in the range 0.1to 0.75 wt. %, with respect to the rubber.

The concentration of hydroxyl groups can be determined using knownmethods, that is, for example, by spectroscopy, titrimetry, elementalanalysis or by determining the so-called hydroxyl value (OH value), thatis by reaction with reagents which release titratable acids on contactwith OH groups. See DIN 53 240 for details.

Rubbers according to the invention with a glass transition temperatureof −120 to −50° C. and 0.1 to 2 wt. % of hydroxyl groups may be usedindividually, blended with aromatic or aliphatic oils or mixed withother rubbers. Additional rubbers which are suitable for the productionof rubber vulcanisates are, in addition to natural rubber, syntheticrubbers. Preferred synthetic rubbers are described, for example, in W.Hoffmann, Kautschuktechnologie, Gentner Verlag, Stuttgart 1980 and I.Franta, Elastomers and Rubber Compounding Materials, Elsevier,Amsterdam, 1989. They include, inter alia,

BR—polybutadiene

ABR—butadiene/C₁-C₄-alkyl acrylate copolymers

CR—polychloroprene

IR—polyisoprene

SBR—styrene/butadiene copolymers with styrene contents of 1 to 60,preferably 20 to 50 wt. %

IIR—isobutylene/isoprene copolymers

NBR—butadiene/acrylonitrile copolymers with acrylonitrile contents of 5to 60, preferably 10 to 40 wt. %

HNBR—partially hydrogenated or fully hydrogenated NBR rubber

EPDM—ethylene/propylene/diene copolymers

and mixtures of these rubbers. For producing vehicle tires, naturalrubber, emulsion SBR and solution SBR rubbers with a glass transitiontemperature above −50° C., polybutadiene rubber with a high 1,4-ciscontent (>90%), which has been prepared using catalysts based on Ni, Co,Ti or Nd, and polybutadiene rubber with a vinyl content of 0 to 75% andtheir mixtures are of particular interest.

Quite specifically preferred rubber mixtures according to the inventioncontain, in addition to the hydroxyl group-containing rubber with aglass transition temperature between −120° and −50° C. additionalrubbers from the group comprising natural rubber, polyisoprene andstyrene/butadiene copolymers with styrene contents between 10 and 50 wt.%. The amount of these additional rubbers is normally in the range 0.5to 95, preferably 40 to 90 wt. %, with respect to the entire amount ofrubber in the rubber mixture. The amount of additionally added rubbersis again governed by the particular ultimate use of the rubber mixturesaccording to the invention.

Rubber mixtures according to the invention contain 5 to 300 parts by wt.of an active or inactive filler such as, for example;

highly disperse silicas prepared e.g. by precipitation from solutions ofsilicates or by flame hydrolysis of silicon halides with specificsurface areas of 5 to 1000, preferably 20 to 400, m²/g (BET surfacearea) and with primary particle sizes of 10 to 400 nm. The silicas mayalso optionally be present as mixed oxides with other metal oxides suchas Al, Mg Ca, Ba, Zn, Zr or Ti oxides,

synthetic silicates such as aluminium silicate or alkaline earth metalsilicates such as magnesium silicate or calcium silicate, with BETsurface areas of 20 to 400 m²/g and primary particle diameters of 10 to400 nm,

natural silicates such as kaolin and other naturally occurring silicas,

glass fibre and glass fibre products (mats, ropes) or glass microbeads,

metal oxides such as zinc oxide, calcium oxide, magnesium oxide,aluminium oxide,

metal carbonates such as magnesium carbonate, calcium carbonate, zinccarbonate,

metal hydroxides such as e.g. aluminium hydroxide, magnesium hydroxide,

carbon blacks. The carbon blacks to be used here are prepared by thelamp black, furnace black or channel black processes and have BETsurface areas 20 to 200 m²/g, such as e.g. SAF, ISAF, HAF, FEF or CPFcarbon blacks,

rubber gels, in particular those based on polybutadiene,butadiene/styrene copolymers, butadiene/acrylonitrile copolymers andpolychloroprene.

Highly disperse silicas and carbon blacks are preferably used asfillers.

The fillers mentioned may be used individually or as a mixture. In aparticularly preferred embodiment, the rubber mixtures contain a mixtureof pale-coloured fillers, such as highly disperse silicas, and carbonblacks, as filler, wherein the mixing ratio of pale-coloured fillers tocarbon blacks is 0.05 to 20, preferably 0.1 to 10.

The fillers are preferably added to the solution of polymerised rubberas solids or as a slurry in water or in a solvent to dissolve thehydroxyl group-containing rubbers(s). The rubber solution may beprepared beforehand, but the solution arising from polymerisation ispreferably used directly. The solvent is then removed thermally or,preferably with the assistance of steam. The conditions for thestripping process can readily be determined in preliminary trials.

Furthermore, the fillers are preferably added to solid hydroxylgroup-containing rubber or a mixture of rubbers and incorporated thereinin a known manner, e.g. with a compounder.

Furthermore, rubber mixtures according to the invention also containoptional cross-linking agents. Sulfur or peroxides may be used ascross-linking agents, wherein sulfur is particularly preferred. Rubbermixtures according to the invention may contain further rubber auxiliarysubstances such as reaction accelerators, antioxidants, thermalstabilisers, light stabilisers, anti-ozonants, processing aids,plasticisers, tackifiers, blowing agents, colorants, pigments, waxes,extenders, organic acids, delaying agents, metal oxides and activatorssuch as triethanolamine, polyethylene glycol, hexanetriol, etc. whichare known in the rubber industry.

In preferred rubber mixtures with highly active precipitated silicas,the use of additional filler activators is particularly advantageous.Preferred filler activators are sulfur-containing silyl ethers, inparticular bis-(trialkyloxysilyl-alkyl)-polysulfides, as are describedin DE 2,141,159 and DE-AS 2,255,577, the oligomers and/or polymers ofsulfur-containing silyl ethers in DE-OS 4,435,311 and EP-A 670,347,mercaptoalkyltrialkoxysilanes, in particularmercaptopropyltriethoxysilane and thiocyanatoalkly silyl ethers such asare described in DE-OS 19,544,469.

The rubber auxiliary substances are used in conventional amounts,governed, inter alia, by the ultimate use. Conventional amounts are e.g.amounts of 0.1 to 50 wt. %, with respect to rubber.

Rubber mixtures according to the invention are outstandingly suitablefor producing moulded items of all types.

Non-restricting examples of these moulded items are O-rings, sections,seals, membranes, tires, tire treads, damping elements and hoses.

Tires and tire treads are particularly preferred.

EXAMPLES Example 1

12.5 g of 1-mercapto-2-ethanol and 1 g of dilauroyl peroxide were addedto a solution of 500 g of solution polybutadiene rubber, Buna CB 65(Bayer AG, Li type, cis-1,4 content about 40%) in 4 l of cyclohexane at70° C. The mixture was then stirred for 8 hours at 80° C. At this pointabout 39% of the mercaptoethanol had reacted. Then 2.5 g of antioxidantVulkanox 4020 (Bayer AG) were added and the solvent was distilled offusing steam (100-110° C.). After drying at 70° C. under vacuum, 508 g ofa colourless rubber with an OH-value of 7, an OH content of 0.21 wt. %and a cis-1,4 content of 40% were obtained. The glass transitiontemperature was −90° C.

Example 2

The same method was used as described in example 1, but only 6.25 g of1-mercapto-2-ethanol were used. After 8 hours at 80° C., 41% conversionwas achieved. Then the solvent was distilled off using steam (100-110°C.). After drying at 70° C. under vacuum, 505 g of a colourless rubberwith a OH content of 0.1 wt. % and a cis-1,4 content of 40% wereobtained. The glass transition temperature was −90° C.

Example 3

12.5 g of 1-mercapto-2-ethanol and 1 g of dilauroyl peroxide were addedto a solution of 500 g of solution polybutadiene rubber Buna VI 47-0(Bayer AG, vinyl-BR, concentration of 1,2-bonded butadiene (vinylcontent) about 47%) in 4 l of cyclohexane at 70° C. The mixture was thenstirred for 4 hours at 80° C. At this point, 95% of the mercaptoethanolhad reacted. Then 2.5 g of antioxidant Vulkanox 4020 (Bayer AG) werethen added and the solvent was distilled off using steam (100-110° C.).After drying under vacuum at 70° C., 512 g of a colourless rubber withan OH value of 17, an OH content of 0.5 wt. %, a vinyl content of about45% and a glass temperature of −54° C. were obtained.

What is claimed is:
 1. Rubbers comprising one or more hydroxyl-groupswhich are produced from diolefins, wherein said rubber(s) contain in therange 0.1 to 2 wt. % of bonded primary hydroxyl groups and have a glasstransition temperature between −120 and −70° C., wherein said rubber(s)has cis-1,4 content, which is polymerized in solution, that is greaterthan 30%, and wherein said rubber(s) have an average molecular weightbetween 50,000 and 2,000,000.
 2. Rubbers according to claim 1, wherein1,3-butadiene and/or isoprene are used as diolefins.
 3. Rubber mixturescomprising rubbers comprising one or more hydroxyl-groups which areproduced from diolefins, wherein said rubber(s) contain in the range 0.1to 2 wt. % of bonded primary hydroxyl groups and have a glass transitiontemperature between −120 and −70° C., and additional rubbers selectedfrom the group consisting of natural rubber, polyisoprene andstyrene/butadiene copolymers with styrene contents between 10 and 50 wt.%, in an amount of 0.5 to 95 wt. % with respect to the entire amount ofrubber in the rubber mixture.
 4. Rubber mixtures according to claim 3,wherein said additional rubbers are present in an amount of 40 to 90 wt.% with respect to the entire amount of rubber in the rubber mixture. 5.A process for preparing rubber mixtures containing in addition torubbers, which comprise one or more hydroxyl-groups which are producedfrom diolefins, wherein said rubber(s) contain in the range 0.1 to 2 wt.% of bonded primary hydroxyl groups and have a glass transitiontemperature between −120 and −70° C., additional rubbers selected fromthe group consisting of natural rubber, polyisoprene andstyrene/butadiene copolymers with styrene contents between 10 and 50 wt.%, in an amount of 0.5 to 95 wt. % with respect to the entire amount ofrubber in the rubber mixture, comprising the step of adding one or morefillers to the solution of rubber(s) in amounts in the range 0.5 to 500parts by wt. with respect to 100 parts by wt. of rubber, and optionally,further auxiliary substances for processing and/or further working-upand/or stabilization are added and then removing the solvent.
 6. Aprocess according to claim 5, wherein the solvent is removed with theassistance of steam.
 7. Molded items comprising rubber mixtures, whichcontain rubbers comprising one or more hydroxyl-groups which areproduced from diolefins, wherein said rubber(s) contain in the range 0.1to 2 wt. % of bonded primary hydroxyl groups and have a glass transitiontemperature between −120 and −70° C., additional rubbers selected fromthe group consisting of natural rubber, polyisoprene andstyrene/butadiene copolymers with styrene contents between 10 and 50 wt.%, in an amount of 0.5 to 95 wt. % with respect to the entire amount ofrubber in the rubber mixture.
 8. A molded item according to claim 7,wherein said molded item is a tire tread or tire sidewall.
 9. Rubbermixtures comprising rubbers comprising one or more hydroxyl-groups whichare produced from diolefins, wherein said rubber(s) contain in the range0.1 to 2 wt. % of bonded primary hydroxyl groups and have a glasstransition temperature between −120 and −70° C., and fillers present inan amount of 0.5 to 5 parts by weight with respect to 100 parts byweight of rubber selected from the group consisting of natural rubber,polyisoprene and styrene/butadiene copolymers with styrene contentsbetween 10 and 50 wt. %, in an amount of 0.5 to 95 wt. % with respect tothe entire amount of rubber in the rubber mixture.
 10. Rubber mixturesaccording to claim 9, wherein said fillers are selected from the groupconsisting of siliceous and carbon blacks or mixtures thereof. 11.Rubber mixtures according to claim 9, wherein said filler is a mixtureof highly dispersed siliceous and carbon black.
 12. Rubber mixturesaccording to claim 11, wherein the mixing ratio of highly dispersedsilicas to carbon black is 0.05 to
 20. 13. Rubber mixture according toclaim 12, wherein the mixing ratio is 0.1 to 10.